Hey all.  I have a 99 impreza outback with 260k; just breaking her in  .  I was on the interstate the other day and i noticed i was slowing down a little, so I gave it more gas and it seemed to bog down and lose power i really had to stomp the gas to get her to go, even when it down shifted i could feel the lack of power. But when I let off the gas a bit i felt the acceleration come back noticeably.   This went on till I got home form time to time. It didn't do it for a couple days then did it again on the hiway, and when I was coming back through town.  It seems to happen when it warms up or has been running for a while.  There is also a hesitation of downshifting now if not coming to a complete stop or giving it gas to pass someone.

 

 I checked the fuel pressure pre-regulator and it was 35, would raise to 40ish when given gas but would drop back to 35 even with throttle open. I replaced the MAF sensor, but it was a cheap one and my car ran horrible with it so I'm guessing mine is ok, unless someone thinks I should buy another one and try it.  I have new plugs, wires, fuelfilter (although I think it may be flowing backwards), checked for vacuum leaks (broke the port on the purge solenoid that runs to the intake manifold), and checked to make sure the knock sensor wasn't cracked or anything.

 

Any help with this would be very helpful as I have a 1700 mile trip coming up in three weeks!

Pull the vacuum hose off of the fuel pressure regulator to check fuel pressure with the engine running. Should get up to about 45 psi and stay there.

 

Also if you can get the pressure guage to a place where you can see it with the hood down, take the car for a drive and watch fuel pressure when accelerating. Make sure it never drops below 35psi. If pressure drops below 35 when accelerating or under load the pump may be going bad.

 

A common cause for trouble like this is a bad Knock sensor. They're cheap on eBay. Under $20 shipped usually. Even if yours isn't cracked it could still be bad.

 

Only thing I would do with the MAF sensor is clean it with MAF or electronics cleaner .

Thanks.  I rented the fp tester from autozone so I had to take it back. 150! but luckily I got it back and the guy that sold me the car said he just changed the knock sensor but it does have small traces of rust on it so who knows.  I figure I'll get the tester back and check it with the regulator hose pulled, if that doesn't work I'll replace the O2 sensor, then knock then throttle position sensor as it really bogs down when its not wanting to downshift like normal. Thanks for the tip on pulling the hose.

knock sensors can be tricky to install correctly, they will crack if over-torqued, the cable must 'dress' away in certain manner or it sits on a bump on some castings causing stress/failure. And I've read that the surface under it needs to be wire brushed.

 

just something to consider

Thanks, I'll go ahead and swap it out for a new one.  20 bucks or so, so no biggie there.  I'm really leaning toward TB posit. sensor now though.  the acceleration always kicks in and out at the same point when giving or letting off the gas no matter the rpms or gear.  The most expensive of the parts, go figure.  But a buddy of mine owns a subaru repair and used car lot so I'll "borrow" one and see what happens 

So I've replaced the fuel filter, MAF sensor, TP sensor, camshaft sensor, O2 sensor, and I'm waiting on a new ECM.  I'm really hoping its the ECM as it still cuts all acceleration after a certain point of applying the gas pedal.  I did notice it runs super smooth and awesome now after all this other than this one intermittent problem.  When I pulled the carpet back in front of the ECM it was wet as hell under there.  Wasn't sure if maybe it could cause a short from all the moister.  Also it seems like when it bogs down and goes lean it doesn't want to down shift at the normal throttle points. Any thoughts would be awesome as I plan on leaving thursday to drive across the country and the ECM doesn't come in till tuesday.  If this doesn't fix it I'm grounded till I can figure it out 

was there any oil on the plug wire boots?

 

maybe a weak coil?

 

a wet area could also have poor/corroded ground connections. Try refreshing ground connections.

Edited March 17, 201412 yr by 1 Lucky Texan

There was no oil on the boots, and I dont think it would be a weak coil since it only happens at a certain point of the throttle opening.  If you think it could be though I'll swap it out.  I did pull my knock sensor and it tested good cool and wasn't cracked, but I need to test it when the problem is happening.

check the timing.

I tested the knock sensor warm and it was erratic. Started at 150 and went down as it cooled.  Also I checked the codes again since I replaced the ecm and O2 sensor.  no more lean condition, just knock sensor.  So I'm really leaning toward that.  How do I go about checking timing or even changing it since everything is controlled by the ECM?  I know the knock sensor highly influences it and that is another nail in that coffin.